But of all the natural disasters which befell the Second Men, the worst was due to a spontaneous change in their own physical constitution. Just as the fangs of the ancient sabre-toothed tiger had finally grown so large that the beast could not eat, so the brain of the second human species threatened to outgrow the rest of its body. In a cranium that was originally roomy enough, this rare product of nature was now increasingly cramped; while a circulatory system, that was formerly quite adequate, was becoming more and more liable to fail in pumping blood through so cramped a structure. These two causes at last began to take serious effect. Congenital imbecility became increasingly common, along with all manner of acquired mental diseases. For some thousands of years the race remained in a
